Live, attenuated strains of bacteria have been successfully used as vaccines to protect humans and animals from disease. The successful vaccine strain is modified to reduce its ability to cause disease to an acceptable level while it retains sufficient pathogenic potential to stimulate a protective immune response in the vaccinated host. Therefore a delicate balance must be achieved as extensive attenuation generally results in reduced immunity.
Shigella is a genus of Gram-negative bacteria belonging to the family Enterobacteriaceae and the etiologic agent of bacillary dysentery or shigellosis, the symptoms of which include abdominal pain, diarrhea, fever, vomiting, and blood or mucus in the stool. Shigella is transmitted by a fecal-oral route, typically through contaminated food or water. As a result, Shigella is more of a public health threat in developing countries where proper sanitation and hygiene are lacking.
The development of an effective live, attenuated strain of Shigella to protect humans against bacillary dysentery (shigellosis) has been hampered by the inability to strike a balance between a need for the strain to invade the intestinal epithelium and reduction of pathogenicity to an acceptable level. Invasive strains that are protective tend to be “reactogenic,” i.e., they cause diarrhea and/or fever in the host.
The current strategy used by most investigators to attenuate Shigella is to mutate or delete genes on the Shigella invasion plasmid in the hopes of reducing the reactogenicity of the vaccine strain while preserving immunogenicity. Levine, M. et al. 2007. Clinical trials of Shigella vaccines: two steps forward and one step back on a long, hard road. Nat Rev Microbial 5:540-553. This strategy is a “top-down” approach starting with a wild type, virulent strain of Shigella. Genes are then systematically inactivated until the strain is sufficiently attenuated to be clinically safe to administer yet still stimulate a robust immune response. For example, the attenuating mutation in S. flexneri 2a strain SC602 is a deletion of virG(icsA) which abolishes the ability of the bacterium to spread from cell to cell after invasion. Coster, T. S. et al. 1999. Vaccination against shigellosis with attenuated Shigella flexneri 2a strain SC602. Infect. Immun. 67:3437-3443. A gene on the invasion plasmid that encodes an enterotoxin and another enterotoxin gene on the chromosome were targets in construction of S. flexneri 2a strain CVD1207 leading to attenuation and reduced reactogenicity. Kotloff, K. L. et al. 2000. Shigella flexneri 2a strain CVD 1207, with specific deletions in virG(icsA), sen, set, and guaBA, is highly attenuated in humans. Infect Immun 68:1034-1039.
A major disadvantage of this “top down” strategy is that the attenuated strains that are protective have the undesirable side effect of causing diarrhea and/or fever. Further, to achieve the appropriate balance in the “top down” approach between preserving immunogenicity and reducing reactogenicity, other genes on the invasion plasmid (also known as the virulence plasmid), and possibly the chromosome as well, that may contribute to reactogenicity must be identified and inactivated.